Electromagnetic and radio frequency impulses can cause disturbances in any electronic equipment. Motion detection circuits used in security systems are sensitive circuits which must respond to weak motion detection sensor signals. RF and EM impulses are able to generate sufficient parasitic responses in motion detection sensors and in their associated motion detection circuits to result in false alarm signals. In the security industry, false alarms are expensive and very undesirable. Each false alarm must be investigated with the same diligence as true alarms. Many false alarms over time degrade confidence in the security system. Most security agencies or city and municipal police forces will charge per inspection fees for investigating false alarms, especially when the security system generates frequent false alarms.
RFI and EMI may result from a variety of sources, such as lightning, radio transmitters and electrical equipment. In motion detectors, shielding the effects of RFI/EMI is conventionally done by providing metal shielding around the detector and its associated circuitry, by designing the printed circuit board carefully to minimize the circuit's susceptibility to RF, by providing short distance wiring for all low level signals, and by using heavy filtering. Shielding is costly and of limited use. While circuit design in the detector can reduce sensitivity to RFI/EMI, both by reducing the amount of parasitic signal received and by reducing sensitivity to "spike" signals, no conventional detector having a good sensitivity to intruder motion is 100% immune from false alarm generation when RFI/EMI noise is added to other acceptable background noise. Consequently, prior art motion detectors, such as passive infrared, active microwave, dual infrared/microwave and ultrasound motion detectors, suffer from the possibility of false alarm generation when subjected to RFI/EMI.